1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device, more particularly to a high brightness nitride semiconductor light emitting device improved in current spreading effect via a novel heterogeneous bonding structure.
2. Description of the Related Art
In general, a nitride semiconductor layer is a group III-V semiconductor crystal such as GaN, InN, and AlN, which is broadly utilized in a light emitting device capable of generating light of short wavelength (ultraviolet rays or green light), especially blue light. The nitride semiconductor light emitting device is fabricated with an insulating substrate such a sapphire substrate or a SiC substrate which satisfies lattice match conditions for the crystal growth. Therefore, typically, the nitride semiconductor light emitting device has a planar structure in which two electrodes coupled to p- and n-nitride semiconductor layers are arranged to face the same direction on the light emitting structure.
Compared to a vertical light emitting device having two electrodes on opposed faces of the light emitting structure, the planar nitride light emitting device is disadvantaged in that an effective area for light emitting is not large-sized due to non-uniform current distribution across the light emitting area and thus light emitting efficiency per light-emitting area is low. FIG. 1 explains the planar light emitting device and limited light emitting efficiency thereof.
FIG. 1 illustrates an example of a conventional nitride semiconductor device 10.
The semiconductor light emitting device 10 shown in FIG. 1 includes a sapphire substrate 11, a buffer layer 12 formed on the substrate 11, an n-type nitride semiconductor layer 14 formed on the buffer layer 12, an active layer 17 formed on the n-type nitride semiconductor layer 14, and a p-type nitride semiconductor layer 16 formed on the active layer 17. A p-electrode 19b is formed on the p-type nitride semiconductor layer 18. Meanwhile, an n-electrode 19a is formed on an exposed area of the n-type nitride semiconductor layer 14 after the p-type nitride semiconductor layer 18 and the active layer 17 are partially removed via mesa-etching.
In this planar semiconductor light emitting device 10, current between two electrodes is concentrated in the shortest route thereof, resulting in a current crowing area A. Also, due to big series resistance caused by horizontal current flow, the planar semiconductor light emitting device 10 exhibits relatively high operating voltage. Disadvantageously, this reduces an effective area for light emitting.
In addition, current crowding causes a great amount of current to be applied in case of a temporary spark or overload. Consequently, heat generated at this time renders the device easily breakable, and thus weakly resistant to electrostatic discharge (ESD). Especially, the n-type nitride area shows lower crystalinity due to high doping concentration of impurities such as Si. Also, the n-type nitride area exhibits extremely low resistance to ESD in the current crowing area A.